Light guides suitable for illuminated displays

ABSTRACT

A light guide ( 1 ) comprises a housing ( 3 ) defining a light-guiding optical cavity having first and second opposed major faces ( 5, 6 ), and a light source ( 11 ) arranged to direct light into the cavity from one end, to be guided between the major faces. The first major face ( 5 ) comprises a window through which light can be emitted from the optical cavity to be used, for example, for illuminating a graphic display. To improve the light distribution across that first major face ( 5 ), the second major face ( 6 ) comprises a sheet material ( 23 ) having a specularly-reflecting surface ( 24 ) that faces into the optical cavity and has diffusely-reflecting light-extraction elements ( 27 ) applied thereto in a predetermined configuration for causing light to be emitted from the optical cavity through the said window.

FIELD OF THE INVENTION

The present invention relates to light guides suitable for use inilluminated displays.

BACKGROUND OF THE INVENTION

It is already known to use light guides to illuminate panels for generallighting purposes and for display applications (e.g. for illuminatingsigns and advertisements, and also for illuminating liquid crystaldisplays). In one form, often referred to as a light box, the lightguide comprises a hollow box-shaped structure defining an opticalcavity, and in another form it comprises a solid light-guiding plate. Inboth forms, a major surface of the guide can be illuminated by lightdirected into the guide in a direction generally parallel to that majorsurface, for example from at least one elongated light source locatedadjacent an edge of the light guide.

Light guides in the form of hollow light boxes are described, forexample, in EP-A-0 490 279; 0 377 309; and 0 293 182; and in GB-A-2 310525. In each of those light boxes, a prismatic optical film is employedwith a view to achieving a more even distribution of light over thesurface that is being illuminated. In addition, an Application Bulletinentitled “Thin Light Box” and issued in March 1990 by Minnesota Miningand Manufacturing Company of St. Paul, Minn., USA describes the designand construction of light boxes, using Scotch™ Optical Lighting Film incombination with a shaped sheet of V-5115 Scotch™ Light Extractor Film,for use in illuminating graphic displays.

In the case of light guides in the form of solid light-guiding plates,it is well known to form light-extraction elements of some type on therear major surface of the plate with a view to achieving a more evendistribution of light over the front surface (i.e. the surface that isbeing illuminated). In some cases, printed light-extraction elements areused and are applied directly to the rear surface of the light-guidingplate. Arrangements of that type are described, for example, in U.S.Pat. Nos. 5,736,686; 5,649,754; 5,600,462; 5,377,084; 5,363,294;5,289,351; 5,262,928; 5,667,289; and 3,241,256.

U.S. Pat. No. 5,618,096 describes light-emitting panels of various typesand mentions the possibility of providing light-extracting deformitieson one or both sides of a panel to control the amount of light emittedfrom any area of the panel. It is also mentioned that the deformitiesmay be printed on a sheet or film which is used to apply the deformitiesto the panel member. WO 92/05535 describes an illuminated display systemcomprising a transparent panel with a dot matrix applied to both sidesand an opaque backing sheet attached to the rear side. An image to beilluminated is printed on a further sheet positioned on the front sideof the panel.

As recognised in previous disclosures, the problems to be addressed inconstructing a light guide for illumination purposes include achieving auniform level of brightness across the panel that is being illuminated,and minimising the amount of power required to produce a level ofillumination that is adequate having regard to the circumstances. Asregards the first of those problems, it is often the case that the panelis more brightly illuminated in the area closest to the light source,which detracts from the overall visual appearance and effectiveness ofthe illumination. As regards the second of those problems, it is clearlyhighly desirable, from an environmental and a cost point of view, thatthe amount of power used for illumination purposes should be kept as lowas possible. Moreover, when the power is derived from a battery (as maybe the case when LCD and computer displays are being illuminated) it isalso generally desirable that the amount of power utilized should beminimized so that the battery can be kept as small and light aspossible.

In addition to those considerations, it would be advantageous to be ableto produce, comparatively easily and in a cost-effective manner, lightguides of widely-differing dimensions that would be suitable for use ina variety of situations but would, nevertheless, function with a highlevel of efficiency.

SUMMARY OF THE INVENTION

The present invention provides a light guide comprising a housingdefining a light-guiding optical cavity having first and second opposedmajor faces, and at least one light source arranged to direct light intothe cavity from one end, to be guided between the major faces; whereinthe first major face comprises a window through which light can beemitted from the optical cavity, and the second major face comprises asheet material having a specularly-reflecting surface that faces intothe cavity and has diffusely-reflecting light-extraction elementsapplied thereto in a predetermined configuration for causing light to beemitted from the optical cavity through the said window.

The term “light extraction element” in this context indicates astructure capable of reflecting light at such an angle that it will beemitted from the optical cavity through the said window. In a preferredform, the light extraction elements are printed elements formed in adiffusely-reflecting material. As used herein, the term “printed”includes the case in which the diffusely-reflecting material isdeposited by a conventional printing process involving temporary contactbetween a printing surface (in which the shape of at least one lightextraction element is pre-defined) and the surface on which the lightextraction elements are to be formed. It also includes the case in whichthe diffusely-reflecting material is deposited by being projected atpredetermined locations onto the surface on which the light extractionelements are to be formed.

Light guides in accordance with the invention can be produced indifferent sizes using comparatively lower cost materials and in a mannerthat is appropriate to high volume production, and can enable theeffective, uniform, and efficient illumination of display panels usingavailable light sources.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, embodiments of the invention will be described withreference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a light guide in accordance with theinvention;

FIG. 2 is a diagrammatic perspective view of a light guide, similar tothat shown in FIG. 1, the light guide being shown partly exploded;

FIG. 3 is a diagrammatic cross-sectional view of the light guide inexploded form on the line III—III of FIG. 2;

FIG. 4 illustrates the rear face of a light guide of the type shown inFIGS. 1 to 3;

FIG. 5 is a graph illustrating a characteristic of the rear face of alight guide of the type shown in FIGS. 1 to 3;

FIGS. 6 and 7 are views, similar to FIGS. 3 and 4, of another lightguide and of the rear face of that light guide;

FIG. 8 is a graph, similar to FIG. 5, for the rear face shown in FIG. 7;and

FIG. 9 illustrates a modification of the light guide of FIGS. 2 and 3.

DETAILED DESCRIPTION

The light guide 1 shown in FIG. 1 comprises a box-like housing 3defining an optical cavity. The housing 3 has opposed major faces 5, 6,and opposed narrow sides 7, 8 and 9, 10. An elongate light source 11 isarranged adjacent one of the narrow sides 7 to direct light into theoptical cavity in a direction generally parallel to the planes of themajor faces 5, 6. One of the major faces (the face 5) forms a windowthrough which light can be emitted from within the optical cavity andused for illumination purposes.

The optical cavity 13 inside the housing 3 is visible in thediagrammatic illustration of FIG. 3. The narrow side 7 of the housingadjacent the light source 11 comprises an optical sheet material 15forming a window through which light from the source 11 can enter thelight guide 1. Preferably, the sheet material 15 has a structuredsurface on the side remote from the light source, to redirect the lightfrom the source 11 and ensure that the light that passes through thiswindow enters the optical cavity 13 preferentially in a directiongenerally parallel to the planes of the faces 5, 6. The optical sheetmaterial 15 may, for example, have a structured surface comprising aseries of ridges and grooves formed by a plurality of paralleltriangular prisms. A similar use of sheet material of that type isdescribed in EP-A-0 293 182. In the light guide 1, the material 15 ispreferably oriented so that the prisms extend parallel to the elongatelight source. Suitable sheet material is available, under the tradedesignation “Scotch™ Optical Lighting Film” from Minnesota Mining andManufacturing Company of St. Paul, Minn., USA.

The narrow side 8 of the light guide 1 opposite the window 15 has areflecting surface 17 on the side facing into the optical cavity 13.This reflecting surface, which is preferably a highly-efficientspecularly-reflecting surface, can be provided by any suitable materialbut is preferably provided by a multi-layer optical film of the typedescribed in U.S. Pat. No. 5,882,774 and WO97/01774. A suitablealternative material is a silver reflective film, for example the filmavailable under the trade designation “Silverlux”, from Minnesota Miningand Manufacturing Company of St. Paul, Minn., USA.

The other two opposed narrow sides 9, 10 of the light guide also havereflecting surfaces 18 facing into the cavity. In this case, thereflecting surfaces are preferably provided by a film materialavailable, under the trade designation “Light Enhancement Film” fromMinnesota Mining and Manufacturing Company of St. Paul, Minn., USA.However, any other suitable reflecting material can be used: generally,it has been found that a diffusely-reflecting material is preferablewhen the length/width ratio of these narrow sides is less than 10 andthat a specularly-reflecting material is preferable when this ratio isgreater than 10. It will be appreciated that this ratio corresponds tothe length/thickness ratio of the light guide 1 (otherwise known as its“aspect ratio”).

The front face, or window, 5 of the light guide comprises an opticalsheet material 19 that, preferentially, guides the light from the source11 along the optical cavity 13 between the faces 5, 6 and permits lightto leave the optical cavity only when it is incident on the material 19at certain angles. More specifically, the sheet material 19 has a smoothsurface facing into the optical cavity and, on the side facing away fromthe optical cavity, a structured surface comprising a series of ridgesand grooves formed by a plurality of parallel triangular prisms wherebylight incident on the material 19 while travelling along the opticalcavity 13 will be totally internally reflected provided it is incidenton the material 19 within a predetermined angular range. As such, thematerial 19 may be the same as the material 15 and, in this case, thematerial is oriented so that the prisms extend in a direction at rightangles to the direction of extent of the light source 11 as indicated inFIG. 2. A similar use of material of that type is described in EP-A-0293 182. To protect the prismatic structures on the sheet material 19, afurther panel 21 may be positioned adjacent the material 19 on theoutside of the light guide housing. This further panel is not essentialbut, when provided, it may comprise a sheet of opalescentlight-diffusing material to enhance even further the uniformity of thelight that passes through the sheet material 19.

The rear face 6 of the light guide 1 comprises a sheet material 23 whichprovides a highly-efficient specularly-reflecting surface 24 facing intothe optical cavity 13. The reflecting surface 24 should be such that itsreflectivity is not reduced substantially for light that is incident indirections other than normal to the surface, and is at least 90%(preferably at least 98%). Preferably, the sheet material 23 is amulti-layer optical film of the type described in U.S. Pat. No.5,882,774 and WO 97/01774. A suitable alternative material, particularlyfor light guides that have a comparatively low aspect ratio (less thanabout 10), is available, under the trade designation “Silverlux”, fromMinnesota Mining and Manufacturing Company of St. Paul, Minn., USA.

As described in greater detail below, the specularly-reflecting surface24 carries diffusely-reflecting light-extraction elements in apredetermined configuration to cause light to be emitted from theoptical cavity 13, through the window 5, in a controlled manner. In thiscase, the light-extraction elements comprise an array of dots 27 formedin a diffusely-reflecting material on the surface 24 as shown in FIG. 4.

In FIGS. 2 and 3, the light source II is shown as being located in athree-sided housing 25, the open side of which is positioned adjacentthe sheet material 15 forming the entry window of the light guide 1. Thehousing 25 is constructed with a view to ensuring that the light source11 directs as much light as possible into the optical cavity 13 and, tothat end, the internal surfaces of the housing may be covered with asuitable highly-efficient, diffusely reflecting material, for example areflective paint or sheet material. Alternatively, the light source 11could be provided with a parabolic reflector to direct the light fromthe source towards the optical cavity 13, or it could be replaced by asuitable apertured light source. The use of the sheet material 15 in thenarrow side 7 of the light guide housing adjacent the light source 11,although preferred, is not essential.

The light guide 1 as described above functions as follows. Light fromthe source 11 (possibly following reflection or redirection at the wallsof the housing 25) enters the optical cavity 13 through the windowmaterial 15 and travels preferentially in a direction parallel to themajor surfaces 5, 6 of the light guide towards the surface 17 where itwill be reflected and returned. However, any light that is incident onthe extraction elements on the rear surface 24 (i.e. the dots 27) willbe diffusely reflected and some of that light will, as a consequence,impinge on the front face 5 of the light guide at such an angle that itcan pass through the optical sheet material 19 and emerge from the lightguide.

The use of light-extraction elements of various forms to cause light tobe emitted from light guides is already well known. In the light guideof FIGS. 1 to 3, the light-extraction elements 27 (as already mentioned)comprise an array of dots formed in a diffusely-reflecting material onthe specularly-reflecting surface 24. The circular form of the lightextraction elements is not essential, however, and they may be of anyshape (for example squares, triangles, lines, etc) that can readily beformed by a printing process, and may even comprise a mixture of shapesand/or sizes. Light-extraction elements in the form of continuous lineson the reflecting surface 24 are also possible. Preferably, thelight-extraction elements 27 are formed by being printed directly ontothe reflecting surface 24 but they could, as an alternative, be printedon a transparent sheet which is then adhered to the surface 24.Moreover, although the use of printed light extraction elements ispreferred, other forms could be employed as described below.

The printed light extraction elements 27 on the reflecting surface 24 ofthe light guide 1 are positioned to provide a required illuminationpattern over the front face 5 of the light guide. In many cases, auniform illumination of the face 5 is required and that can be achievedif the percentage area of the surface 24 that is covered by thediffusely-reflecting elements 27 varies (most typically, increases) withthe distance from the light source 11 (measured in the direction atright angles to the direction of extent of the light source). That isillustrated diagrammatically in FIG. 4, in which it will be seen thatthe proportion of the surface 24 of the sheet material 23 that iscovered by the light extraction elements 27 is zero in the regionimmediately adjacent the light source 11 and then increases as thedistance from the light source increases. In FIG. 4, the surfacecoverage of the light extraction elements 27 is shown reaching a maximumvalue at a short distance from the other end of the sheet 23 and thendecreasing slightly in the region furthest from the light source 11.This decrease is provided to accommodate the effects of the reflectivesurface 17 at the far end of the optical cavity 13; the need for it (andits extent), will be determined in each case by the particularconfiguration of the light guide. It should be understood that FIG. 4 ispurely diagrammatic and that the variation in the surface-coverage ofthe light-extraction elements 27 would typically be continuous ratherthan discontinuous as shown in this drawing. A more typical variation inthe surface-coverage of the light-extraction elements 27 is illustratedin FIG. 5 which shows that the surface coverage is zero over the first10% of the length of the optical cavity 13 measured from the lightsource 11 and then increases linearly, reaching 100% (i.e. totalcoverage) at a distance of about 90% of the length of the optical cavitymeasured from the light source. The surface coverage then decreasesslightly at the end of the optical cavity 13 remote from the lightsource 11.

FIGS. 6 and 7 illustrate a light guide 31 that is generally similar tothe guide illustrated in FIGS. 2 to 4 but utilises an additional lightsource 11′ positioned opposite to the light source 11 (i.e. adjacent thenarrow side 8 of the housing 3). To enable light from the source 11′ toenter the optical cavity 13, the side 8 of the housing 3 comprises anoptical sheet material 15′ forming a window, rather than the reflectingmaterial 17 of FIG. 3. In addition, the rear surface 24 of the lightguide is provided with a suitably-modified configuration oflight-extraction dots 27′, described in greater detail below.

The light source 11′ is located in a three-sided housing 25′ similar tothat of the light source 11 but, like the light source 11, it couldalternatively be provided with a parabolic reflector to direct lightfrom the source into the optical cavity, or be replaced by a suitableapertured light source. The material 15′ forming the window from thehousing 25′ into the optical cavity 13 is preferably the same as theoptical sheet material 15.

The light guide 31 functions in a similar manner to the guide 1described above except that, in this case, light from both sources 11,11′ (possibly following reflection or redirection at the walls of theassociated housing 25, 25′) enters the optical cavity 13 through theassociated window material 15, 15′ and travels preferentially in adirection parallel to the major surfaces 5,6 of the light guide towardsthe light housing at the other end of the optical cavity where some ofthe light will be reflected and returned. Any light that is incident onthe extraction elements on the rear surface 24 (i.e. the dots 27′) willbe diffusely reflected and some of that light will, as a consequence,impinge on the front face 5 of the light guide at such an angle that itcan pass through the optical sheet material 19 and emerge from the lightguide.

As with the light guide 1 of FIGS. 2 to 4, the printed light extractiondots 27′ on the reflecting surface 24 of the light guide 31 arepositioned to provide a required illumination pattern over the frontface 5 of the light guide. In many cases, a uniform illumination of theface 5 is required and that can be achieved if the percentage area ofthe surface 24 that is covered by the diffusely-reflecting dots 27′varies (most typically, increases) with the distance from each of thelight sources 11, 11′ (measured in the direction at right angles to thedirection of extent of the light sources) up to a maximum in the centralregion equidistant from both light sources. That is illustrateddiagrammatically in FIG. 7, in which it will be seen that the proportionof the surface 24 of the sheet material 23 that is covered by the lightextraction elements 27′ is zero in the regions immediately adjacent thelight sources 11, 11′ and then increases in each case as the distancefrom the respective light source increases, reaching a maximum value inthe central region 33 of the surface. It should be understood that FIG.7 is purely diagrammatic and that the variation in the surface-coverageof the light-extraction dots 27′ would typically be continuous ratherthan discontinuous as shown in this drawing. A more typical variation inthe surface-coverage of the light-extraction elements 27′ is illustratedin FIG. 8 which shows that the surface coverage is zero over the first10% of the length of the optical cavity 13 measured from each of thelight sources 11, 11′ and then increases linearly, reaching 100% (i.e.total coverage) in a central region at a distance approaching 40% of thelength of the optical cavity measured from each light source.

FIGS. 4 and 7 both indicate that the surface-coverage provided by thelight-extraction elements 27, 27′ is varied by changing the number ofdots per unit area of the surface 24 while maintaining a uniform dotsize. As an alternative, the size of the extraction elements can bechanged while maintaining a constant number of extraction elements perunit area of the surface 24 and, as a further alternative, both the sizeof the extraction elements and the number per unit area can be varied.

In some cases, it may also be appropriate to vary the surface-coverageof the light extraction elements 27, 27′ transversely of the opticalcavity (i.e. in a direction parallel to the direction of extent of thelight source(s) 11, 11′).

The extraction elements 27, 27′ of FIGS. 4 and 7 can be printed usingany suitable printing medium that will function as a diffuse reflectorand is compatible with the reflecting surface 24 and with the printingprocess employed. One suitable medium is a highly-efficientdiffusely-reflecting matt white ink. Any suitable printing process canbe used to deposit the printing medium on the surface of the sheetmaterial, including screen printing, gravure printing, offset printing.Ink jet printing may also be employed. In a preferred process, theprinting medium is deposited using a conventional silk screen printingprocess because that is a versatile, comparatively low-cost processwhich is suitable for long production runs but nevertheless enables goodcontrol to be maintained over the size of the extraction elements 27,27′. The extent to which the reflecting surface 24 should be covered bythe printing medium can be determined empirically for a particular lightguide by providing an arbitrary, linearly-varying, pattern of extractionelements 27, 27′ on the surface 24 and comparing the resultingillumination of the front face 5 of the light guide with that achievedin the absence of any extraction elements. The pattern is then adjusted,on the basis of that comparison to yield the illumination required.

Although it is generally required to achieve a constant level ofillumination across the front face 5 of the light guide 1, 31 there maybe occasions when it is desirable to provide a level of illuminationthat varies across the face 5 in a predetermined manner. For example,the level of illumination across the front face 5 could be matched tothe image that is being illuminated so that the brighter parts of theimage receive more light and the darker parts of the image receive less.That could be achieved, for example, by first providing, on thereflecting surface 24, the pattern of extraction elements 27, 27′required to provide uniform illumination of the front face 5 andsubsequently superimposing, on that pattern, further extraction elementsarranged in an image-dependent configuration. The further extractionelements could, for example, be printed directly over the elements 27,27′ or they could be printed on a separate transparent sheet which isthen adhered to the already-printed surface 24. As an alternative, thefurther extraction elements could be provided on the smooth surface ofthe optical sheet material 19 in the front face 5 of the light guide, inwhich case they should be formed of a translucent material, capable ofboth reflecting and transmitting incident light from within the opticalcavity 13. As a further alternative, the extraction elements 27, 27′could be omitted from the reflecting surface 24 (at least in certainareas) with only the further, image-related, extraction elements beingprovided.

An arrangement of the type referred to in the previous paragraph isillustrated diagrammatically in FIG. 9, for a light guide intended to beilluminated by a single light source 11 as in FIGS. 2 and 3. Thereflecting surface 24 carrying the pattern of extraction elements 27required to provide uniform illumination of the front face of the lightguide is shown, in combination with a sheet 35 carrying the graphicimage to be illuminated. Superimposed on the reflecting surface 24 is atransparent sheet 37 carrying further extraction elements 39 arranged inan image-related configuration whereby the brighter parts of the imagereceive more light and the darker parts of the image receive less.Typically, the graphic image on the sheet 35 is a digitally-printedimage, in which case the image data file that is used to print the imagecan also be used to print the extraction elements 39 on the sheet 37taking account, if required, of factors such as the nature of the inksused in the graphic image and the spectral sensitivity of the human eye.

Although FIG. 9 shows the further extraction elements 39 located on theseparate sheet 37, it will be understood that they could alternativelybe located on the reflecting surface 24. In that case, when using adigital printing process, all of the required light extraction elements(i.e. elements 27 as well as elements 39) can readily be deposited onthe surface 24 together.

The use of a sheet material 23 for the rear face of the optical cavity13 of the light guides 1, 31 is advantageous because such a material iseasy to handle, not only during the actual printing process (when thefact that the sheet material has flat, unstructured, surfaces is aparticular advantage) but also during any subsequent drying and storagestages prior to assembly of the light guide. When in use in the lightguide, the reflective sheet material 23 prevents light from leaving theoptical cavity 13 through the rear face 6 and thus enhances theillumination of the front face 5. In addition, any scratches on thesurface of the reflective sheet material (which might arise, forexample, during handling or assembly of the light guide) will notadversely affect the illumination of the front face 5. Moreover, becausethe extraction elements 27 are printed on a planar reflecting surface 24there is also no risk of more interference patterns arising even whenthe extraction elements are disposed in a regular array.

The printing medium used to form the extraction elements 27, 27′ isselected for compatibility with the sheet material to which it isapplied, as well as for its durability and diffusely-reflectingcharacteristics. Highly-opaque white inks are preferred and it has beenfound that the best printing quality is obtained using UV-cured inks ina screen printing process. Screen printing offers the further advantagethat the ink layer deposited on the sheet material 23 is thick and,consequently, comparatively robust and also less likely to fade anddiscolour. Moreover, unlike some printing processes, the screen printingprocess does not entail the application of high pressures to thematerial 23 and is less likely to damage the latter. It can also be usedto apply the printing medium to a wide range of different sheetmaterials in a wide range of sizes. It should be understood, however,that other media could be employed to form the light extraction elements27, 27′, as could other processes including, for example, laserprinting, ink jet printing, thermal transfer printing and thermal inkjet printing.

In some cases, the extraction elements 27, 27′ may be formed by othermethods, including, for example, surface roughening of the sheetmaterial or deposition of diffusely-reflecting material (which mayinclude particles) in a required configuration using coating or sprayingprocesses.

A hollow light guide as described above with reference to FIGS. 1 to 3or 6 can be fabricated in such a way that it is comparativelylightweight. That is a particular advantage when the light guide islarge in size (for illuminating large signs, for example), andespecially when it is required to be installed in a less accessiblelocation. It also makes the use of thicker light guides (which offer thepossibility of admitting more light into the optical cavity 13) morepractical. It is also comparatively simple to fabricate light guides ofthis type in many different sizes and, in particular, with widelydiffering aspect ratios (i.e. different length/thickness ratios). Forexample, light guides of this type can be produced with aspect ratios assmall as 5 and as large as 100 and, in both cases, will functionefficiently. Light guides with small aspect ratios offer the advantagethat the light admitted to the optical cavity 13 undergoes fewerreflections before it is emitted through the window 5. Consequently, thelevel of accuracy required in the configuration of the light extractionelements 27, 27′ is lower.

Of particular interest in the field of illuminated signs is the factthat light guides of the type shown in FIGS. 1 to 3 and 6 can befabricated with widths as small as 10 cm and even, depending on the sizeof the sign, as small as 3 cm. Light guides of this type, having lowaspect ratios (typically less than 10) and using conventionalfluorescent tubes as light sources, have been found to have performanceefficiencies that compare favourably (and, in some cases, veryfavourably) with those that can be achieved using solid light guides. Inthe alternative case in which the light guides have larger aspectratios, they are found to be better able than solid light guides toaccommodate some degree of misalignment of the light source.

The light sources employed with the light guides 1, 31 are not requiredto have an elongate form as illustrated. Other light sources could beemployed including, for example, light emitting diodes (LEDs) Dependingon the form of the light source, more than one source may be used todirect light into the optical cavity 13 through the adjacent side of thehousing 3. In that case, the surface-coverage of the light extractionelements 27, 27′ may also vary transversely of the optical cavity (i.e.between the sides 9, 10).

The light guides illustrated in FIGS. 1 to 3 and 6 have been describedabove as being used to illuminate a graphic display but they could beused for other purposes including, for example, to illuminate liquidcrystal displays.

An example of an illuminated sign incorporating a light guide of thetype illustrated in FIGS. 1 to 3 will now be described.

The housing 3 of the light guide 1, excluding the front major face 5,may be a one-piece vacuum-formed construction of any suitable material,for example PVC (polyvinylchloride). Alternatively, the housing may beformed from several pieces of, for example, an acrylic material, eachproviding one side of the housing, which are secured together in anysuitable manner. The housing is approximately 60×60×4.5 cm.

The internal surface of the rear major face 6 of the housing is coveredwith a sheet 23 of a specularly-reflective, multi-layer, optical film ofthe type described in U.S. Pat. No. 5,882,774 and WO 97/01774, having areflectivity in a direction normal to the surface of the film of atleast 98%. The surface 24 of the film 23 facing into the housing 3carries a printed dot pattern providing a percentage surface coveragethat varies as represented in FIG. 5. The dot pattern was screen printedon the surface 24 of the film 23 using a white UV-cured ink of a typeformulated for the printing of compact discs (a suitable ink beingavailable from NOR-COTE of Eastleigh, Hampshire, England). The variationin percentage surface coverage of the surface 24 by the ink was achievedby varying the size of the dots while maintaining a constant number ofdots per unit area of the surface (based on transverse lines of dotscontaining about 20 dots per inch (2.54 cm)).

The internal surface of one narrow side 7 of the housing 3 is coveredwith a sheet 15 of the above-mentioned “Scotch™ Optical Lighting Film”,arranged with the prisms facing into the housing and extending parallelto the long edges of this side of the housing. The internal surface ofthe opposite narrow side 8 of the housing 3 is covered with the samespecularly-reflective film material as the internal surface of the rearmajor face 6 but without the printed dot pattern. The internal surfacesof the remaining two narrow sides 9, 10 of the housing 3 are coveredwith the above-mentioned “Light Enhancement Film”.

The housing 3 is closed with a sheet 19 of the above-mentioned “Scotch™Optical Lighting Film”, forming the front major face 5. The film isarranged so that the prisms are on the outside of the housing and extendbetween the narrow sides 7 and 8.

The light guide module thus formed was put into a sign housing andprovided with a 60 cm long, 14W fluorescent lighting tube located,within a high-reflectance housing 25, adjacent the narrow side 7 of thelight guide housing 3 and arranged to direct light into the latter. Itwas found that the front major face 5 of the housing 3 was illuminatedwith a high degree of uniformity and to a level sufficient to provideeffective illumination of a graphic image located in front of the face5.

1. A light guide comprising a housing defining a light-guiding optical cavity having first and second opposed major faces, and at least one light source arranged to direct light into the cavity from one end, to be guided between the major faces; wherein the first major face comprises a window through which light can be emitted from the optical cavity, and the second major face comprises a sheet material having a specularly-reflecting surface that faces into the cavity and has diffusely-reflecting light-extraction elements applied thereto in a predetermined configuration for causing light to be emitted from the optical cavity through the said window.
 2. A light guide as claimed in claim 1, in which the specularly-reflective surface has a reflectivity of at least 90% for light incident on the surface at any angle.
 3. A light guide as claimed in claim 1, in which the sheet material having the specularly-reflecting surface is laminated to an internal surface of the housing.
 4. A light guide as claimed in claim 1, in which the light-extraction elements are printed elements formed in a diffusely-reflecting material.
 5. A light guide as claimed in claim 4, in which the light-extraction elements are printed directly on the specularly-reflecting surface.
 6. A light guide as claimed in claim 1, in which the percentage area of the specularly-reflecting surface that is covered by light extracting-elements is not constant over the whole area second major face.
 7. A light guide as claimed in claim 6, in which, in any region of the second major surface, the percentage area of the specularly-reflecting surface that is covered by light extracting-elements varies with the distance of that region from the light source.
 8. A light guide as claimed in claim 1, including a second light source arranged to direct light into the cavity from the end opposite the first-mentioned light source, to be guided between the major faces.
 9. A light guide as claimed in claim 1, in which the first major face has a structured surface comprising a plurality of parallel prisms on the side remote from the optical cavity.
 10. A light guide as claimed in claim 1, in which a display that is to be illuminated is positioned in front of the window.
 11. A light guide as claimed in claim 10, in which at least some of the, diffusely-reflecting light-extraction elements are applied to the specularly-reflecting surface in a configuration that is related to the display.
 12. A light guide as claimed in claim 10, including diffusely-reflecting light-extraction elements applied to the window, on the side facing into the optical cavity, in a configuration that is related to the display.
 13. A light guide as claimed in claim 1, wherein said light source is an elongate light source arranged adjacent an edge of said housing.
 14. A light guide as claimed in claim 1, wherein said housing includes an optical sheet material adjacent said light source that forms a window through which light from the light source can enter the light guide.
 15. A light guide as claimed in claim 14, wherein said optical sheet material adjacent said light source has a structured surface on the side remote from the light source.
 16. A light guide as claimed in claim 1, wherein said optical cavity first major face comprises an optical sheet material having a smooth surface facing into the optical cavity and a structured surface comprising a series of ridges and grooves facing away from the optical cavity.
 17. A light guide as claimed in claim 1, wherein said specularly-reflective surface has a reflectivity of at least 98% for light incident on the surface at any angle. 